Process for strengthening a low carbon high strength steel



July 8, 1969 CHIAKI AsADA ETAL 3,454,432

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CARBON HIGH STRENGTH STEEL Filed May 2, 1966 Sheet of '7 '0--). 2 a 4 15 /rmanewf ffm/'w United StatesN Patent O U.S. Cl. 148-12.?) 5 Claims ABSTRACT OF THE DISCLOSURE The yield point is increased without appreciable change in toughness and ductility for a martensitic steel containing 0.10-0.25% carbon and not more than 7% of an interstitial alloying constituent such as silicon, manganese, nickel, chromium, molybdenum, vanadium, titanium and mixtures thereof, by cold-working such steel at ambient temperatures to create therein a strain value up to 3.0% and then subjecting the strained steel to an aging treatment at low temperatures.

This invention relates to a novel process for strengthening a low carbon high strength steel to obtain the strengthened steel of which yield point or proof stress is equal to or higher than that of a medium carbon high strength steel, without decreasing the original toughness and ductility of the low carbon high strength steel. More particularly, this invention relates to a novel process for strengthening a low carbon high strength steel compris-l ing 0.10 to 0.25% by weight of carbon, not more than 7% by Weight in total of at least one kind of metal selected from the group consisting of silicon, manganese, nickel, chromium, molybdenum, vanadium, titanium and the alloying elements, and, the balance, iron including incidental impurities and having the martensitic structure as quenched or as tempered at a temperature not higher than 350 C. after quenching, characterized by causing the steel to create therein not more than 3% permanent strain and then subjecting the strained steel to an aging treatment in the range of temperatures'at which the strained steel will not be decreased in strength to obtain the strengthened steel having an improved yield point or proof stress without decreasing the original toughness and ductility of the steel. i

In this connection, it is more desirable for the low carbon high strength steel to contain at least two kids of metals than to contain only one kind of metal selected from said group, from the view-point of the hardenabjllity of said starting steel as well as the mechanical properties and production cost of a strengthened steel to be obtai'ed from said starting steel.

The steel produced by the process of this invention can be used as a material of, for example, bolts, gears and axles.

Generally speakin-g, there are two different ways to allow a common structural steel to have more than 100 kgJmm.2 of tensile strength. One way is to make a sorbitic steel of the common structural steel by adding thereto elements such as molybdenum, vanadium andthe like which will provide tempering resistance and secondary hardening, and the other one is to make a tempered martensitic steel of a low-alloy or medium-alloy 'steel by tempering at a lower temperature. It is generally known that such a tempered martensitic steelhaving a specially lower carbon content has excellent toughness 'and ductility even when the steel is high in strength. A

3,454,432 Patented July 8, 1969 ice low carbon martensitic steel, however, has generally some disadvantages that the yield ratio is somewhat lower than that of a medium carbon sorbitic steel, and, especially that it will be remarkably decreased if the carbon content be below about 0.2% by weight. There are some ways such as aging treatment to eliminate such disadvantages as mentioned above.

However, in a low carbon martensitic steel, age hardening cannot be expected with substitution-type alloying elements but necessarily depends upon interstitial alloying ones contained in the steel which are capable of di'using in the steel at a comparatively low temperature. Furthermore, with reference to the low carbon martensitic steel as quenched or tempered at a lower temperature, sufcient eifects cannot be obtained merely by low-temperature aging treatment to improve yield ratio because of a usually small amount of the interstitial alloying elements which form a supersaturated solid solution of the elements in the matrix. Accordingly, in addition to the aging treatment, some other supplementary means are necessary to employ in attaining a satisfactory increase in the yield ratio.

As a result of our basic experiments on the aging of a steel, we have found that, in the case of a martensitic steel, the best way of all of various ones to carry out eiectively the aging by use of the interstitial alloying elements is to cause the steel to have therein an inner energy (a strain) by cold working before the aging treatment.

We have further confirmed that a low carbon martensitic steel may be comparatively easily given therein a strain uniformly, because said steel does not show its own definite yielding phenomena like an austenite steel.

Thus, the present invention has been accomplished on the basis of the above observations of the experiments.

An object of this invention is to provide a novel strengthened steel product having an improved yield point or proof stress without any decrease in the usual toughness and ductility of a low carbon martensitic steel.

Another object of this invention is to provide a novel process for strengthening a low carbon high strength steel to produce the novel strengthened steel.

A technical effect of this invention is as follows:

The application of this invention to a low-carbon low-alloy or a low-carbon medium-alloy high strength steel having a martensitic microstructure as quenched or as tempered at low-temperature will not deteriorate said steel in toughness and ductility, but improve it in yield point and proof stress, and this fact means that said steel can be enhanced in yield ratio to such an extent that said steel equals or surpasses in the yield ratio a medium carbon high strength steel having tempered sorbitic structure.

This invention will be concretely explained hereunder, by referring to the attached drawings.

FIG. 1 shows relations between mechanical properties and carbon contents of a low carbon high strength steel such as a 0.8 Si-1.2 Mn-l.5 Cr steel;

FIG. 2, relations between mechanical properties and tempering temperatures of the said Si-Mn-Cr steel;

FIG. 3, relations between mechanical properties and low-temperature tempering times of the steel in which a permanent strain has been created after the quenching of the steel;

FIG. 4, relations between notch strength ratios and the permanent strains of the same steel;

FIG. 5, relations between mechanical properties and not more than 1% permanent strains of the steel further tempered;

FIG. 6, relations between hardness, Charpy impact 3 values and permanent strains of the same steel as represented in FIG. 6;

FIG. 7, the stress-strain diagram of the same steel at different strain levels; and

FIG. 8, relations between mechanical properties and not more than permanent strains of the same steel. FIGS. 3-8 are specially intended for the illustration of the effects of this invention.

Now, refer to the figures.

As will be seen from FIG. 1 in which there are shown relations `between the mechanical properties and carbon contents of 0.8 Si-1.2 Mn-l.5 Cr steel (as one example of low carbon high strength steel) which has been tempered at 300 C. after the oil quenching at 880 C. (880 C. O.Q.), the tensile strength and proof stress of the steel remarkably increase in a linear way and the yield ratio of the steel also increases with the increase of the carbon content thereof, While the yield ratio will decrease to less than 0.8 in lower carbon range in which the toughness (elongation and impact value) of the steel may still be kept at a satisfactory level. Such tendencies as the above have also been clearly found in the properties of a low carbon Mn-, Cr-Moor Ni-Cr-Mo high tensile structural steel. FIG. 2 in which relations between the mechanical properties and tempering temperatures of said Si-Mn-Cr steel are shown by curves indicates that, in the same way as in FIG. 1, the proof stress of the steel increases very sharply as the tempering temperature rises up, reaches the maximal value of about 125 kg./mm.2 at about 300 C. and then begins to decrease gradually as the tempering temperature rises. The yield ratio, however, gradually increases with the rising of the tempering temperature, While the yield ratio obtainable in the range of tempering temperatures (275-325 C.) is no more than a value of 0.80-083, which is somewhat lower than the yield ratio 0.9 of a tempered sorbitic steel obtained by tempering it at about 500 C.

However, as shown in FIG. 3, a low carbon high strength steel (0.19 C-0.76 Si-1.29 Mn1.52 Cr*0.08 Ti), similar in composition to the preceding one, which has been tempered at 200 C. for not more than 5 hours after the creation of 0.2% permanent strain in the steel by tension at ambient temperature will be remarkably improved in 0.2% offset proof stress and U notch Charpy impact value simultaneously with tensile strength, reduction of area and elongation being left unchanged, as compared with one which has been treated in the same way as the above except for the creation of the permanent strain. This fact, therefore, means the yield ratio of the former steel has been strikingly enhanced. It has been eX perimentally confirmed that almost the similar results are obtainable even when the low-temperature tempering temperature employed is 300 C. Then, test pieces each having a notch of stress concentration factor 4.0 Were subjected to a tension test after they had given not more than 1.0% permanent strain in order to investigate the effect of the notch on a strain aging.

As shown in FIG. 4, the results are that the 0.2% proof stress is remarkably enhanced and that the notch strength ratio can be kept at a level as high as about 1.3.

In the practice of such a strain aging of a steel, the steel is sometimes left untreated for an unduly long time after having been quenched, for mere reasons of operational arrangements. Because, in such a case, an aging crack is likely to happen, the steel is necessary to temper immediately after the quenching in order to avoid such a crack. Thus, what effects such a strain aging would have on the properties of the steel which had been quenched and then tempered was investigated. As shown in FIG. 5, the result is that a low carbon high strength steel (0.17 C-0.67 Sie-1.18 Mn-1.34 Cr), similar in properties to said steel, which had been quenched at a temperature of 885 C., subjected to a low-temperature tempering at 300 C. to allow the steel to have a tensile strength of about 100 kg/mm, caused by tension at ambient temperature 4 to create therein not more than 1.0% permanent strain uniformly and retempered at 200 C. for 1.5 hours, was strikingly increased in 0.2% olset proof stress simultaneously with tensile strength, reduction of area and elongation each remaining at almost the same level. Accordingly, the thus-treated steel was remarkably increased in yield ratio to a value of about 1.0. It has been experimentally confirmed that the similar result is obtainable for a steel of the similar composition which has been treated to have a tensile strength of -160 kg./mm.2. As shown in FIG. 6, the impact value slightly decreases with the increase of the strain of which value is 1% at highest. The similar tendency was found in the steel which has been treated to have a tensile strength of 150-160 kg./rnm.2.

The effect of this invention is clearly seen in FIG. 7 in which the stress-strain diagrams of the same steel at different strain levels are represented.

In adrltion, the test pieces for tension test subjected to the treatments of this invention were so broken that the fracture looked like a shallow cup in the same way as in a sorbite steel. This means that the pieces will not easily cause brittle fracture in spite of their high yield ratio.

As is apparent from FIG. 8, the magnitude of a permanent strain given in a steel should be a value of 3.0% and below, in order to obtain a satisfactory effect of the strain on the steel; while a permanent strain of a value of more than 3% is not desirable, because such strain will be far less effective to increase yield point and will, on the contrary, decrease toughness and ductility as well as impact value.

A low carbon martensitic steel with not more than 0.2% carbon content, in its low-temperature tempered state, has shown a somewhat lower yield ratio, which is a weak point of the steel, while a steel subjected to the treatments including the strain aging of this invention is characterized by being substantially freed from such weak point and hardly decreasing in toughness and ductility even if the steel has a remarkably higher yield ratio and higher level of strength.

EXAMPLE 1 TABLE 1.-TREATING CONDITIONS Quenehing Strain given Tempering Group A 885 C. x 30 0.2% permanent 200 C. x 0, 0.5,1.0,

min., O.Q,. strain given. 1.5, 3.0 and 5.0 hr., O.T. Group B do No strain given.-.. Do.

O.T. Oil tempering.

The mechanical properties of the treated steel bars of Group A and B are shown in Tables 2 and 3.

Remarks: O.Q.=Oil quenching.

TABLE 2.MECHANICAL PROPERTY [0.2% otset proof stress, kg./mm.2]

Tempel-ing time (hr.) 0 0. 5 1. 0 1. 5 3.0 5.0

Group A (Strain given) 142. 8 135. 7 133. 0 132. 2 Group B (No strain given) 97. 7 114.3 113. 5 115. 2 116. 5 118. 0

TABLE 3.-MECHANICAL PROPERTY [U notch impact value, kgm./crn.2]

Tempering time (hr.) 0 0. 5 1. 0 1. 5 3. 0 5.0

Group A (Strain given) 8. 3 9. 4 9. 3 9. 3 9. 4 9.1 Group B (No strain given) 7.6 7. 8 7. 7 8. 3 7. 5 7. 9

The mechanical propertes are shown in more detail u1 FIG. 3.

TABLE 4.-MECHANICAL PROPERTIES Permanent strain, percent 0. 2 0.4 0. 6 0. 8 1. 0

0.2% otset proof stress (kg/mm!) 71. 6 89. 0 94. 8 99. 0 100. 5 102.3 19. 7 19. 4 19. 2 19. 0 18.8 17. 7

Elongation (percent) The mechanical properties are shown in more detail in FIGS. and 6.

What we claim is:

1. A process for strengthening a low carbon high strength steel having a martensitic structure and comprising (L10-0.25% by weight of carbon, not more than 7% by weight in total of at least one kind of metal selected from the group consisting of silicon, manganese, nickel, chromium, molybdenum, vanadium, titanium and other alloying elements, and, the balance, iron including incidental impurities, characterized by cold-working at ambient temperature the steel to create therein not more than 3.0% permanent strain and then subjecting the strained steel to an aging treatment in the range of temperatures at which the strained steel will not be decreased in strength to` obtain the strengthened steel having an improved yield point or proof stress without deterioration of toughness and ductility.

2. A process for strengthening a low carbon high strength steel having a tempered martensitic structure which has been tempered at not more than 350 C. and comprising OJO-0.25% by weight of carbon, not more than 7% by Weight in total of at least one kind of metal selected from the group consisting of silicon, manganese, nickel, chromium, molybdenum, vanadium, titanium and other alloyng elements, and, the balance, iron including incidental impurities, characterized by cold-Working at ambient temperature the steel to create therein not more than 3.0% permanent strain and then subjecting the strained steel to an aging treatment in the range of temperatures at which the strained steel will not be decreased in strength to obtain the strengthened steel having an improved yield point or proof stress without deterioration of toughness and ductility.

3. A process as set forth in claim 2 wherein the cold- Working is a drawing treatment.

4. A process as set forth in claim 3 wherein the aging treatment is carried out at temepratures below the tempering temperature.

5. A process as set forth in claim 1 wherein the permanent strain is between 0.2 and 1.0%.

References Cited UNITED STATES PATENTS 3,388,011 6/1968 Zackay et al. 148-123 L. DEWAYNE RUTLEDGE, Primary Examiner.

W. W. STALLARD, Assistant Examiner. 

